1. Field of the Invention
The present invention relates to a gas sensor for detecting a trace amount of a gas component, and in particular it is related to a gas sensor which detects a decomposed gas resulting from decomposition of SF.sub.6 due to a discharge which has taken place inside of a gas-insulated electrical apparatus.
2. Description of the Related Art
FIG. 13 shows an exemplary conventional gas sensor, which is described in Japanese Patent Laid-Open No. 62-207952. In this figure, a detecting electrode 1 is made of a mixture of a metal halide and a metal ion conductive solid electrolyte, and is so structured that the gas to be detected is allowed to be contacted with it. A solid electrolyte 2 is a solid conductor of metal ions. In this illustrative example, the metal is Ag, the metal ion is Ag.sup.+, and the solid electrolyte is a solid conductor of Ag.sup.+ ions. A counter electrode 3 is made of a mixture of a metal halide, a metal and a metal ion conductive solid electrolyte. Lead wires 5-1 and 5-2 attached to each electrode 1 and 3, are connected to a voltmeter having a high internal resistance.
The operation of the gas sensor structured as above and applied to a gas-insulated electrical apparatus is explained below. In the above-mentioned publication, the gas component resulting from decomposition of SF.sub.6 by discharge is explained to be F.sub.2 but generally it is considered to be HF. Assuming that the gas produced by the discharge is HF, the following reaction takes place on the detecting electrode 1:
2HF (on the detecting electrode)+2 Ag.sup.+ (on the solid electrolyte)+2e.sup.- (on the metal).fwdarw.2AgF+H.sub.2
On the other hand, the following reaction takes place on the counter electrode 3.
2AgF (on the counter electrode).fwdarw.F.sub.2 (on the counter electrode)+2Ag.sup.+ (on the solid electrolyte)+2e.sup.- (on the metal).
The composition is so set that the activity (partial pressure) of F.sub.2 generated on the counter electrode 3 becomes constant. The electromotive force between the detecting electrode 1 and the counter electrode 3 is represented by the following equation according to the Nernst equation. EQU E=A+B log P.sub.HF
wherein, E: the electromotive force PA1 A, B: constants
P.sub.HF : the partial pressure of HF on the detecting electrode side.
Accordingly, if the relation between the partial pressure of HF (P.sub.HF) and the electromotive force (E) is obtained previously, the partial pressure of HF, that means that the concentration of the decomposed gas generated by discharge, can be obtained by measuring E at the time of the measurement.
One of the problems of the conventional gas sensor having the above-mentioned features is that it cannot be used repeatedly since the detecting electrode 1 material reacts with the gas to be detected and forms a reaction product. That means that, when the hydrogen fluoride (HF) which is the gas resulting from the decomposition by discharge is detected with the detecting electrode 1 which is made of silver (Ag), as the resulting silver fluoride (AgF) is a stable compound, even when the concentration of the gas to be detected (HF) is lowered, the output voltage is not decreased correspondingly, but keeps the signal level of the highest gas concentration. Therefore it has been impossible to carry out the measurement which attributed to the decrease of the concentration.
Also, since the output is generated as a result of the natural reaction between the gas to be detected (HF) and the electrode active substance (Ag.sup.+ ion) on the electrode, the time required for the reaction is long. For instance, sometimes the response time becomes some hours or more. Therefore, the detection can not be done quickly, and this has been another problem of the conventional gas sensor.